In a crystal oscillation circuit, a crystal oscillator starts vibrating when a voltage is applied to the crystal oscillator. As the amplitude of the vibration gradually increases with time and the vibration is stabilized, the crystal oscillation circuit is made available to an external circuit. That is, the crystal oscillation circuit requires a start-up time.
Japanese Patent Application Publication No. 2009-188738 describes a method for exciting a crystal oscillator using a voltage-controlled oscillator to shorten a start-up time. However, there is a need to match the frequency of the voltage-controlled oscillator, provided externally, with the resonance frequency of the crystal oscillator according to trimming, temperature correction, etc. When the Q value of the crystal oscillator is high, it is difficult to apply the method.